Urea grease has achieved rapid growth as a grease that may be conveniently applied to typical grease-lubricated applications including a wide variety of bearings for vehicle constant-velocity joints, ball joints, wheel bearings, alternators, cooling fans, ball screws, linear guides of machine tools, a wide variety of sliding areas of construction equipment, and bearings and gears in steel equipment and various other industrial mechanical facilities.
The usage of urea grease has been rising steadily in particular applications, such as various kinds of vehicle parts including constant-velocity joints (CVJs) where there is a strong demand for durability and reduced friction and wear in sliding areas in response to the trend of the present time toward miniaturization, weight reduction, and a hostile use environment, and in steel equipment that requires a highly heat-resistant, wear-resistant lubricating grease.
In addition to the individual components that make up a urea grease, another contributing factor to the final properties and characteristics of the urea grease is the particular process and conditions under which the urea grease is manufactured. Process conditions, for example, the dispersing and mixing of the individual components and temperature variations may be significant factors affecting the urea grease structure, for example, the nature of the fibers formed.
U.S. Pat. No. 5,314,982 to Christian Rasp et al. discloses a three-part process, which can also be employed on a large industrial scale, for the preparation of polyurea lubricating greases, which is characterised (a) in that polyureas which carry oleophilic groups are prepared by reaction of diisocyanates with amines either in the absence of a solvent on a reaction screw at 80°-120° C., preferably at 85°-95° C., (variant A) or in a toluene medium at 20°-80° C., preferably at 30°-60° C., (variant B), (b) in that, after complete reaction, the polyureas prepared according to (a) are ground in the solid, dry state to give powders (at least 70% by weight of the powder having particle sizes of about 100-400μ) and (c) in that the ground crude product, after being made into a “paste” (wetted) at elevated temperature in the base oil employed (10-30 minutes at 140°-180° C.) and cooled again to room temperature, is processed to a grease by complete homogenisation—if appropriate in several passes—in a high-pressure homogeniser under a pressure of 400-1,500 bar (spontaneous heating occurring up to 100° C.), greases having readily reproducible and essentially the same properties as in the case of the previously customary in situ manufacture being produced.
U.S. Pat. No. 4,392,967 to A. Gordon Alexander discloses a process for continuously manufacturing a lubricating grease using a screw process unit comprising: (a) introducing feed materials and lubricating oil into selected locations of a screw process unit which contains a series of adjacent, longitudinally connected barrel sections for performing different operative steps and houses a rotating screw device traversing the interior of the barrel sections and having separate elements along its length to perform desired operations; (b) mixing and conveying said feed materials along said process unit through the adjacent barrel sections by continuous operation of said rotating screw; (c) controlling the temperature of said material while it is being conveyed through said process unit by use of various heat exchange means which are located in or adjacent each barrel to aid in carrying out the operative steps of dispersion, reaction, dehydration and/or homogenization; (d) venting water resulting from the dehydration of the feed mixture at selected barrel discharge points in said process unit; (e) introduction of additional lubricating oil and/or additives at downstream barrel locations following the dehydration step; (f) homogenization of said complete grease formulation by continued rotation of said screw device; and (g) removal of the finished lubricating grease from the end barrel section of said screw process unit.
Despite the history of urea greases and processes of preparing urea greases, there is still a need for a continuous process of preparing a urea grease that will provide advantages regarding process, process stability, quality control, and economy. There is also still a need to provide for a process of preparing a urea grease that will provide for the flexibility to be applicable for many types of urea greases without significant changes in the equipment being used.